1. Field of the Invention
This invention relates to methods and apparatus for treating airborne residues and, more particularly, to a safe application of ozone to the airborne perchloroethylene vapor that is emitted from dry cleaning processes in order to convert the vapor into harmless substances for release to the atmosphere, and the like.
2. Description of the Prior Art
There is a need to protect the atmosphere from degradation in quality through contaminants that are released in the course of any number of industrial processes. Illustratively, with increasing population and a matching growth in industrial activity, the discharge into the atmosphere of contaminants which are attributable to these sources is swiftly reaching and, in some cases, may already have exceeded the earth's natural capacity to absorb pollutants of this nature. As a consequence, many industries are taking measures to reduce the burden that their products and processes had heretofore imposed on the environment. Environmental protection laws have also been enacted not only to encourage those industries that have thus far failed to adopt environmental protection programs to nevertheless take necessary action, but also to establish some tangible standards for acceptable emission levels.
Typical of these industrial processes is the process for dry cleaning clothing in which the garments that are to be cleansed are treated with perchloroethylene, a volatile organic solvent. After cleansing with perchloroethylene, and largely as a matter of process economics, most of the perchloroethylene solvent and vapor is recovered to be used again. These perchloroethylene recovery techniques, however, reclaim only about 98% of the solvent and vapor. The remaining 2% of the solvent is often released to the atmosphere as an un-recoverable vapor. Clearly, there is a need for an efficient and economical system that can treat most, if not all, of the otherwise escaping perchloroethylene vapor in order to prevent this vapor from becoming an environmental contaminant.
A number of proposals for suitably treating potentially harmful industrial compounds that otherwise would discharge into the environment have been advanced through the years. Typical of these proposals are:
U.S. Pat. No. 5,205,927 granted Apr. 27, 1993 to G. B. Wickramanayake for "Apparatus For Treatment of Soils Contaminated With Organic Pollutants" which discloses an apparatus for generating ozone, that is, an unstable oxygen molecule of three oxygen atoms, O.sub.3, in contrast to the usual, stable two oxygen atom (O.sub.2) molecule. It should be noted, in this respect, that extremely small quantities of ozone occur in the atmosphere through normal environmental processes. This patent describes generating artificially produced ozone, however, in a gas that is applied to contaminated soil in order to decompose the contaminating organic compounds that are in the soil.
U.S. Pat. No. 5,151,197 granted Sep. 29, 1992 to A. J. Smith et al. for "Fluid Treatment" shows an apparatus for treating liquid wastes with oxygen, oxygen enhanced air or ozonated air.
U.S. Pat. No. 5,123,176 granted Jun. 23, 1992 to K. Yamada et al. for "Method And Apparatus For Dry Cleaning As Well As Method And Device For Recovery Of Solvent Therein" is directed to a dry cleaning device in which the solvent gas is reused.
U.S. Pat. No. 4,640,782 granted Feb. 3, 1987 to J. C. Burleson for "Method And Apparatus For The Generation And Utilization Of Ozone And Singlet Oxygen" discloses an apparatus for generating ozone and singlet, or "nascent" oxygen (O.sub.1). Nascent oxygen is composed of single oxygen atoms, unattached to any other substance. Oxygen in this form is highly unstable and swiftly combines, usually with another oxygen atom, to form a stable O.sub.2 molecule. The ozone and nascent oxygen are, as stated in this patent, mixed with pathogens to kill the pathogens.
U.S. Pat. No. 4,572,821 granted Feb. 25, 1986 to E. Brodard et al. for "Apparatus For Dissolving Ozone In A Fluid" shows a device for mixing an ozonized gas with a fluid and from which a mixture of fluid and ozonized gas is discharged.
U.S. Pat. No. 4,204,955 granted May 27, 1980 to E. T. Armstrong for "System For Pollution Suppression" is directed to an apparatus for increasing the ozone concentration in a gas and mixing the gas with fluid waste.
U.S. Pat. No. 4,035,301 granted Jul. 12, 1977 to E. T. Armstrong for "System For Pollution Suppression" shows a surge suppression apparatus and a gas purifying scrubber.
Although several of these foregoing proposals suggest the use of ozone to convert organic matter into environmentally compatible compounds, none of these proposals suggest a way in which ozone can be produced and applied with safety, efficiency and economy to an organic vapor in order to reach this result.
In this respect, as noted above, oxygen, in its usual, stable state occurs generally in a molecule of two oxygen atoms. Through appropriate treatment, e.g., application of a high voltage electrical field to a supply of oxygen, or some other oxygen bearing gas, limited quantities of ozone molecules are produced.
With specific emphasis on perchloroethylene vapor treatment, ozone combines through a sequence of chemical reactions with the perchloroethylene to produce carbon dioxide (CO.sub.2), water (H.sub.2 O), more oxygen (O.sub.2) and some simple chloride compounds. All of these reaction products are environmentally acceptable. Further in this respect, although the additional oxygen (O.sub.2) creates a somewhat fresher atmosphere and the air quality also is improved because airborne bacteria, odors and the like are destroyed, nevertheless great care must be exercised in connection with the ozone that is generated.
For example, prolonged or continued exposure to ozone in concentrations of 0.10 parts per million (ppm) is toxic. Because, as mentioned above, very small concentrations of ozone occur naturally in the atmosphere, humans and animals do have a tolerance, or acceptance level for this gas. Thus, it has been learned that concentrations of ozone of 0.05 ppm or less have no toxic effect on human beings or on animals. Because the ozone molecule is so unstable, it also has been learned that these molecules will separate in not more than about thirty minutes after formation into the usual, stable oxygen molecule of two oxygen atoms (O.sub.2), the third oxygen atom from the ozone molecule linking up with any other molecule that can accept it.
Prior art ozone generators also were subject to a rapid accumulation of oxidized particles on the plate or tubular ozone generator surfaces to an extent that generator operation between maintenance cleanings was reduced to a few dozen hours. To make matters even more difficult, unbolting and removing these plate or tubular generator surfaces for cleaning was a tedious and time-consuming business, requiring the fouled generator tubes or plates to be withdrawn individually from the ozone generator. This, of course, aggravated the short operational span that characterized these generators by imposing lengthy maintenance periods.
These ozone generators were also subject to a number of additional disadvantages. For instance, there is the problem of isolating the ozone generator from potentially explosive vapors. There is the further problem of ozone generation efficiency. Prior art ozone generators, for example, used high volume air bypass apparatus for ozone production. More than 95% of the process air in these bypass systems did not come into contact with the electrical fields that produced the ozone, but flowed over the outside of the plates or tubes that were used to establish these electrical fields. This produced a very low conversion rate (less than 1/100 of one percent of the available oxygen molecules were converted into ozone molecules) and the requirement for large, inefficient and bulky apparatus with high volume air handling capacities to generate the needed ozone concentration. These prior art ozone generators were also unsatisfactory because the air handling and ozone generation capacity for each device was inflexibly fixed. In this circumstance, a different size ozone generator was required to meet each separate commercial air handling and ozone production requirement.
Accordingly, there is a need for a safer and more efficient and economical apparatus for generating and applying ozone to various substances in order to convert these substances, of which perchloroethylene is typical, into environmentally acceptable compounds. More specifically, there is a need for an improved ozone generator that has greater conversion efficiency with a lower volume of process air. There is, moreover, a further need for an ozone generator that is readily adaptable to different air handling and ozone concentration requirements.